During drilling operations, a circulating fluid such as a drilling fluid is typically circulated from the surface through a drill string down hole to the drill bit and returned to the surface through an annulus defined between the drill string and the borehole, which may be cased or open hole. The drilling fluid is circulated in order to cool and lubricate the drill bit and to permit the removal of rock cuttings and other debris from the borehole as it is being drilled. In addition, the drilling fluid may be utilized as a means for controlling the formation pressures and stresses during drilling, such as by providing a desired pressure in the borehole, and to thereby inhibit undesirable events such as blowout, fracture of the formation or borehole collapse.
More particularly, it may be desirable during drilling operations to maintain a pressure in the borehole which is greater than the adjacent formation pressure to prevent a blowout and influx of fluids from the formation onto the borehole. However, if the borehole pressure exceeds the fracture pressure of the formation, a formation fracture may occur. Thus, the formation fracture pressure typically defines the upper limit for allowable borehole pressure in an open or uncased borehole.
Often, the exposed formation in an open borehole immediately below or down hole of the lowermost portion of a casing string, such as an intermediate casing string, will tend to have the lowest fracture pressure in the open borehole. However, the lowest fracture pressure may occur at greater depths in the open borehole.
Changes in the borehole temperature caused by drilling operations and/or the passage of circulating fluids through the borehole may alter or affect the effective fracture gradient of a formation. The fracture gradient is the pressure per unit depth required to fracture or cause the rock of the formation to separate. For instance, circulation of drilling fluid may result in a temperature of the drilling fluid down hole which is lower than the static geothermal temperature, with the result that the drilling fluid may have a cooling effect on the surrounding formation. This cooling effect may reduce the near borehole formation stresses by thermal contraction and subsequent cracking of the rock wall and may result in a lower effective fracture gradient. Lower effective fracture gradients in turn may increase the likelihood of the occurrence of undesirable events such as formation fracture, lost circulation and borehole collapse.
For example, significant cooling of drilling fluid may occur prior to reaching sensitive or vulnerable sections of the borehole in deep water rigs where a large volume of drilling fluid resides in a riser section located between the rig on the ocean surface and the wellhead on the ocean floor. This riser section may be thousands of feet in length. For example, some deep water rigs in the Gulf of Mexico are now exceeding 10,000 feet of ocean depth. Thus, the drilling fluid in the riser section tends to be significantly cooled by the ocean. The cooling effect is especially pronounced during long time intervals between circulation events, such as when tripping drill pipe out and back in the borehole. During this time, the ocean water can cool the drilling fluid in the riser section through thermal conduction to near freezing temperatures. Further, if tripping drill pipe into the borehole, the drilling fluid in the drill pipe is also cooled, even if drilling fluid is added into the drill pipe from the surface during the operation.
Given that the drilling fluid in the riser section is not heated prior to circulation, when circulation does finally commence, this relatively cold drilling fluid proceeds down hole and eventually contacts the open hole borehole without having an opportunity to be warmed adequately by thermal conduction from the formation heat before exiting the bottom of the drill string. Attempts have been made to heat the drilling fluid on the surface prior to being conducted down hole. However the thermal conduction heat losses tend to be quite high and heat continues to be lost as the drilling fluid propagates through the riser section and below the ocean floor.
Accordingly, it would be desirable to minimize the cooling effect of a circulating fluid on an open borehole. Minimizing the cooling effect may increase the effective fracture gradient, thereby reducing the likelihood of undesirable events such as formation fracture, lost circulation and borehole collapse and thereby potentially reducing the number of casing strings required in order to drill and complete the borehole.
It would also be desirable to provide an apparatus for heating a circulating fluid in order to minimize the cooling effect of the circulating fluid in the borehole. It would be especially desirable to provide such an apparatus which heats the circulating fluid in the borehole so that heat energy provided to the circulating fluid is not dissipated before the circulating fluid reaches vulnerable sections of the borehole.